Study of Massive OBA Stars with X-ray Emission

We have studied the physical parameters of a group of 15 OBA stars from which X-ray emission was detected by the SRG/eROSITA telescope. Whereas the X-ray emission from cool FGKM stars is directly produced near their surface zones-in their chromospheres and coronae-the nature of the X-ray emission from OBA stars requires a separate study in each specific case, since single OBA stars are not intrinsic X-ray sources. We have determined the main physical characteristics of the stars under study, including their effective temperature \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{ extrm{eff}}$$\end{document} and surface gravity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\log g$$\end{document}, based on the spectral energy distribution and the optical spectra obtained with the 1.5-m Russian-Turkish RTT-150 telescope. An additional analysis of the H\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\alpha}$$\end{document} line profiles has allowed the possible X-ray emission mechanisms to be assessed: generation in non-stationary stellar winds, collisional processes in circumstellar envelopes, and coronal emission of hidden cool companions. As a result of the analysis, we have concluded that the X-ray emission from eight stars with typical X-ray luminosities in the range \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\log L_{ extrm{X}}=28.5{-}30.0$$\end{document} is determined by their hidden late-type companions.